View Single Post
Old 08-28-2020, 06:16 PM   #16 (permalink)
Master EcoModder
JulianEdgar's Avatar
Join Date: Nov 2017
Location: Australia
Posts: 2,001
Thanks: 106
Thanked 1,572 Times in 1,116 Posts
Originally Posted by aerohead View Post
1) '... the shape of a body in front of the largest cross-section has only minor influence on the total drag. The main contributions to the drag force originate from the rear part of the body.... it is very important to design a rear body surface which brings the divided streamlines smoothly together. Optimum shapes are 'streamlined' bodies having a very slender rear part.' Hucho
2) ' ... the optimum shape in terms of drag is a half-body, which forms a complete body of revolution together with its mirror image- produced through reflection from the roadway.' Hucho
The normal Aerohead irrelevancies. None of the above has anything to do with fineness ratio.

3) ' OPTIMUM FINENESS RATIO' is presented in Hoerner, Page 70.
4) Maximum aft-body contraction geometry necessary to prevent flow separation is given by Mair and Buchheim et al. in Hucho. ( 22-23 degrees)
5) The lowest drag streamline body which satisfies the aft-body contour limit is a 2.5:1 fineness ratio, provided by Hoerner, and illustrated in Hucho's drag table (derived from Hoerner's data ), page 61, TABLE 2.1, 3rd from bottom.
You're looking at the 'aerodynamic streamlining template.'
By definition, this shape provides the lowest drag, three-dimensional flow, half-body, free of flow separation, with minimum surface friction and pressure drag. It's a known quantity. A sure thing. Defined by Hucho, with supporting evidence by same.
I've given you all this information before.
Yes, Aerohead has written all this before. The trouble is, all this is basically ignored (1 -2 pages max in a whole book, if that) by all the current major authoritative texts on automotive aerodynamics. Even with vehicles where you could argue it could be of significance (solar race cars), the most authoritative book on the subject (Tamai) doesn't even have 'fineness ratio' in the index.

Why do they ignore it? Because it's of so little significance. To listen to Aerohead, you'd think these books would be dominated by the subject.

To restate my original response:

1) I don't understand the point of the tool - how does it help people actually achieve anything?

2) There seems to be a lot of oversimplification happening here - eg "Since drag is directly related to fineness-ratio". No, that's not the case on any real-world car.

3) Aerohead says: "The fun begins when trying to figure how one shape beats another." I am afraid I think that is just rubbish.
  Reply With Quote